eyelidsvaries

Eyelids vary across individuals and species in structure, appearance, and function. In humans, they protect the eyes, spread the tear film, and shield the ocular surface from debris and light. The eyelid anatomy includes skin, the orbicularis oculi muscle, the levator palpebrae superioris, the tarsal plate, and glands such as the Meibomian glands. Eyelashes and the lacrimal caruncle are additional surface features that contribute to protection and tear dynamics.

In humans, eyelid appearance is influenced by ethnicity and age. Some people have a pronounced eyelid crease (often termed a double eyelid), while others have a single eyelid or an epicanthal fold. The presence or absence of folds, along with eyelid shape and eyelid margin contour, contributes to a range of eye shapes described as hooded, almond, or rounded. Newborns may have partially fused eyelids, and creases typically develop with growth. Aging can lead to skin laxity, drooping (dermatochalasis), and changes in eyelid contour and function.

Common eyelid conditions include blepharitis, an inflammation of the eyelid margins; meibomian gland dysfunction; ptosis, or drooping of the upper lid; and lid malpositions such as entropion or ectropion. These conditions vary in prevalence and severity and can affect comfort, appearance, and vision.

Beyond humans, eyelid structure varies across the animal kingdom. Many mammals possess a third eyelid or nictitating membrane for additional protection and tear distribution. In some species, eyelids are reduced or oriented differently to suit their ecological needs, while birds and reptiles may rely on alternate protective membranes and blinking patterns.